Of late, economical and lightweight greenhouses, pool coverings and other similar structure are constructed with corrugated composite or plastic sheets which have a high strength to weight ratio, a high transparency and a strong resistance to the elements. Normally, such sheets are constructed in long but relatively narrow extruded corrugated strips. The strips are cut in the desired length for a particular installation and then once installed on a framework, are lap joined together side-to-side to form an enclosure. Such lap joints have been formed using screws with elastomeric washers backed up by lap fasteners which must be held on one side of the panels by a first installer while the screw is inserted from the other side by a second installer. The economies of such structures are predicated on their ability to be erected quickly with little labor and the requirement for two workers to install the lap joint fasteners is a major installation expense which is heretofore been unavoidable.